Lithium-containing Na–Fe-amphibole from cryolite rocks of the Katugin rare-metal deposit (Transbaikalia, Russia): chemical features and crystal structure

Detailed chemical and structural studies were carried out for Li–Na–Fe-amphibole from cryolite rocks of the Katugin deposit, Transbaikalia. The rocks contain 30–70 vol.% cryolite, mafic minerals as Fe-silicates (Li–Na–Fe-amphibole, Li-containing fluorannite, and bafertisite), oxides (magnetite, ilmenite, pyrochlore, cassiterite, and others), and sulfides (sphalerite, pyrite, and chalcopyrite). Quartz, K-feldspar, polylithionite, REE-fluorides, and albite occur as minor or accessory phases. The chemical composition of amphibole (wt.%) varies as follows: SiO₂, 48.5–48.9; TiO₂, 0.4–0.8; Al₂O₃, 1.6–2.2; Fe₂O₃, 15.9–17.1; FeO, 17.6–18.4; MnO, 0.8–0.9; ZnO, 0.3–1.1; MgO, 0.2–0.3; CaO, <0.1; Na₂O, 8.4–8.7; K₂O, 1.4–1.5; Li₂O, 0.6–0.8; H₂O, 0.7–0.8; and F, 2.2–2.5. The amphibole has a specific composition intermediate among the F–Fe members of the Na-amphibole subgroup: 40–45 mol.% ferro-ferri-fluoro-nybøite, 40–45 mol.% ferro-ferri-fluoro-leakeite, and 10–20 mol.% fluoro-riebeckite ± fluoro-arfvedsonite. The mineral is monoclinic, space group C2/m, a = 9.7978(2), b = 17.9993(3), c = 5.33232(13) Å, β = 103.748(2)º, V = 913.43(3) ų, and Z = 2. The structural formula of Li–Na–Fe-amphibole is (Na_0.46K_0.24□_0.30)Na_2.00(⁠$Fe0.952+$Mg_0.05)₂⋅(⁠$Fe0.953+$Ti_0.025Mg_0.025)₂(Li_0.37$Fe0.482+$Mn_0.10Zn_0.05)[(Si_0.91Al_0.09)₄Si₄O₂₂](F_0.58(OH)_0.42)₂. Raman and Mössbauer spectroscopy data are given for this amphibole.